1. Field of the Invention
The present invention relates to methods and apparatus for injection molding preforms so that their subsequent reheating and blow-molding into containers is simplified. In particular, the present invention relates to a method and apparatus for providing an improved neck-ring or neck split components of an injection mold that allows for an earlier ejection or removal of the preform from the injection mold, thus reducing time needed to manufacture the preform. The method and apparatus are particularly well-suited for thermoplastic polyester polymer materials such as polyethylene terephthalate.
2. Related Art
Well known by those skilled in the art, the preform is a tube with a generally hollow circular cross-sectional configuration having a body portion, a closed end portion with a generally hemispherical configuration, and an open end. About the open end and superimposed between the open end and the body portion is a generally circular neck-finish. Ultimate container needs will dictate specific details of preform size and shape. Although smaller and larger sizes are feasible, technicians make specific preform configurations for specific container configurations with a capacity typically between 250 ml to four liters.
For receiving a closure (i.e., a lid), the neck-finish has a configuration generally having a sealing surface portion adjacent to the open end, a handling ring portion adjacent to the body portion that helps facilitate manufacture of the blow-molded container, and a threaded portion between the sealing surface and handling ring for attachment of the closure. To assure proper closure attachment and seal, the neck-finish requires sufficiently consistent and accurate dimensional characteristics generally free of distortions or deformations. While a screw thread is a common form, the threaded portion can be any form of lugs, snap-rings, or other appendages for attaching the closure, such as, but not limited to, a standard crown neck finish.
Also well known by those skilled in the art is the injection molding process. The process involves injecting a thermoplastic polymer or other plastic material at a molten elevated temperature through a small opening or nozzle into the injection mold. The injection mold is an assembly of various components creating a closed and sealed cavity that allows the molten polymer to form the preform without leakage between components. Once the injected polymer material sufficiently cools and solidifies, selected components of the injection mold separate to allow preform ejection or removal.
In a commonly used process for blow molding the container, an oven of a blow-molding machine heats and softens the polymer material of the body portion of the preform but not the neck-finish. The blow-molding machine, holding the preform by the handling ring portion of its neck-finish, places the heated preform into a blow-mold cavity where pressurized air then inflates and expands to conform the preform to the blow-mold cavity, thus forming the container. The neck-finish configuration of the blow-molded container generally remains unchanged and retains the configuration acquired when initially injection molded as the preform.
The time needed to injection-mold the preform is typically limited by the time needed to cool and solidify injected polymer material sufficiently to permit removal of the part from the mold without causing deformation or distortion. Usually, a segment of the preform having a thicker wall cross-sectional dimension determines the cooling time required. The plastic within the thicker wall cross-sectional segment generally requires more time to cool and solidify sufficiently and the neck-finish often has one of the thicker wall cross-sectional segments.
To form the open end and hollow circular cross-sectional configuration of the preform, the injection mold assembly typically uses a core component that is a substantially straight-sided rod with a longitudinal axis. Surrounding and adjacent to the core component is the neck-ring or the neck split components. The neck-ring is a pair of semicircular pieces that accurately shape the dimensional characteristics of the neck-finish and assists in removing the preform from the core component.
During preform removal, an apparatus within the injection mold causes the neck-ring components to initially move in unison in a direction parallel to the longitudinal axis of the core rod. The neck-ring components bearing against the threaded portion and handling ring portion of the neck-finish cause the preform to slide in a longitudinal direction from the core component.
Molten thermoplastic polymer material at its elevated temperature will generally shrink as it cools and solidifies. Accordingly, in manufacture, the preform will generally shrink against the core component as the material cools. As the core component restrains the shrinkage, molecular forces develop that cause the preform to grip the core's side. Forces acting on the threaded portion and handling ring (support ledge) portion of the neck-finish during removal must transmit through the wall of the preform to overcome frictional resistance created by the grip of the preform against the core. In other words, the forces applied to the threaded portion and the handling ring (support ledge) portion of the neck-finish are in shear with the resistance of the grip of the preform against the core.
The polymer material does not solidify at the same moment. Generally the material in direct contact with mold surfaces will solidify sooner than material not in direct contact. If the polymer material has not sufficiently solidified throughout the neck-finish wall cross-section, the neck-finish will not have sufficient strength to transmit the force and thus can deform and distort during removal, causing the sealing surface portion to become irregular and incapable of maintaining proper seal with the closure. Consequently, molding technicians extend cooling time to assure polymer solidification of the neck-finish, thus preventing distortion. For thermoplastic polyester polymer materials, the time typically needed to inject and cool the polymer and remove the preform is about 21 to 26 seconds.
Thus, in most preform designs, the portion limiting the earliest stripping time is the neck finish portion. FIG. 1 is a cross-sectional view of a preform mold assembly 10 having a core cooling channel 12, a core cooling tube 14, a neck-ring cooling channel 16, a neck-ring or neck split components 18a and 18b, a core component 20 having an axis 21, a mold cavity block 22 with a cavity surface 23, and a mold cooling channel 24 which extends circumferentially around the mold cavity block 22. FIG. 1 also shows a preform 26, a mold gate insert 28, and an injection nozzle 30. The preform mold assembly 10 is an assembly of various components that creates a closed and sealed cavity that allows molten polymer injected into the cavity to form the preform 26 without substantial leakage between components. In FIG. 1, the preform 26 has a configuration that is substantially identical to the closed cavity.
The core-cooling channel 12 includes a cooling inlet 32 and a cooling outlet 34. The neck-ring components 18a and 18b mount to the ejector bar 36a and 36b, and slide respectively on a wear pad 38 by a means of cams and gibs (not shown). The wear pad 38 fastens to a stripper plate 40. A core holder 41 retains the core component 20. The preform 26 has an open end 50, a closed end 52, a body portion 54, and a neck-finish 44. The neck-finish 44 has a sealing surface portion 45, a threaded portion 46, and a handling ring portion (support ledge) 48. The neck-ring components 18a and 18b comprise a pair of semicircular pieces that accurately shape the dimensional characteristics of the neck-finish 44 and assist in removing the preform 26 from the core component 20.
During the preform 26 removal or ejection, the preform mold assembly 10 initially separates along a parting line 42 allowing the core component 20, the core holder 41, the neck-ring components 18a and 18b, the preform 26, and other associated components to move in unison in a direction parallel to the axis 21 and thereby pull the preform 26 free from the mold cavity block 22, the mold gate insert 28, and the nozzle 30, thus separating the preform 26 from the cavity surface 23. Actuation of the stripper plate 40 then causes the ejector bar 36a, 36b and the neck-ring component 18a, 18b to initially move in unison in a direction parallel to the axis 21 to remove the preform 26 from the core component 20. Eventually, the neck-ring component 18a and the ejector bar 36a move in a first direction perpendicular to and away from the axis 21 on the wear pad 38 and simultaneously the neck-ring component 18b and the ejector bar 36b move in a second and opposite direction (of that taken by the neck-ring component 18a and the ejector bar 36a) perpendicular to and away from the axis 21 on the wear pad 38 setting the preform 26 entirely free from the preform mold assembly 10.
In addition to the distortion problem described above, another problem with known mold designs is where the neck ring halves do not seal against the core when they are closed (assembled), and the mold is then closed and clamped. After the mold has been opened and the part is ejected, the neck ring halves 18a and 18b that are carried forward by the stripper plate 40 are separated from each other. Before the next molding cycle can commence, the ejection mechanism must be reversed to restore the neck rings and stripper plate to their molding positions, shown in FIG. 1. This reversing procedure includes moving the neck rings towards each other until they touch during the backward stroke of the stripper plate so that, by the time the stripper plate has fully returned (in the position shown in FIG. 1), the neck rings are completely closed with their mutual parting surfaces touching. The complete closing of the neck rings can be performed at any point during the stroke of the return of the stripper plate as the neck rings are not in any danger of touching the core at any point.
In designs where the neck rings are going to touch the core in the mold closed position, it is preferable that they themselves are first closed so that when they finally touch the core they do so as an assembled pair. In the case of an earlier Husky design, the neck rings had a “shut-off” cylindrical surface that was parallel to the longitudinal axis of the core and touched the core diameter. However, this design is not optimal because if there is a gap between these two cylindrical surfaces greater than about 0.001 inch, the risk of plastic leaking through this gap during injection is significant. Consequently, this type of design requires close tolerance manufacture of these surfaces to ensure the assembled gap is minimized. Unfortunately, molds wear as they are used, and eventually a design like this leaks. Another early Husky design had a tapered, or conical, shut-off surface that contacted a correspondingly mating tapered surface on the core. These two surfaces were pressed together during molding, causing a positive seal that prevents plastic leakage. However, this design was not optimal because the preform still had neck-ring distortions when it was stripped from the core.
FIG. 2 is a partial cross-sectional view of selected components shown in FIG. 1 and further showing the preform 26 having a wall thickness 56, and the core component 20 having a core surface 58. The mold cavity block 22 (not illustrated in FIG. 2) has separated from the neck-ring 18b along the parting line 42.
FIG. 3 is a partial cross-sectional view similar to FIG. 2. The neck-ring 18b has initially moved in direction “A” parallel to the axis 21 to begin removal of the preform 26 from the core component 20. The neck-ring 18b (and 18a, not illustrated in FIG. 3) has separated from the core holder 41 along a sub-parting line 64. Furthermore, the preform 26 has partially separated 59 from the core surface 58. The sub-parting line 64 ends at the neck-finish 44 adjacent to and between the sealing surface portion 45 and the threaded portion 46 (see FIG. 2).
Molten thermoplastic polymer material at its elevated temperature will generally shrink as it cools and solidifies. Accordingly, in manufacture, the preform 26 will generally shrink against the core component 20 as the material cools. As the core component 20 restrains the shrinkage, molecular forces develop that cause the preform 26 to grip the core surface 58. Forces acting through neck-ring 18b (and 18a, not illustrated in FIG. 3) and ultimately bearing on the threaded portion 46 and the handling ring portion 48 of the neck-finish 44 during removal must transmit through the wall thickness 56 of the preform 26 to overcome friction created by the grip of the preform 26 against the core surface 58. If the polymer material has not sufficiently solidified throughout the neck-finish wall thickness 56, it will not have sufficient strength to allow transfer of forces to overcome friction of preform sticking around the core component 20 at about a point 60 of the core surface 58. This in turn will cause neck-finish distortion 62 as the neck-ring 18b (and 18a, not illustrated in FIG. 3) move in direction “A.” The distortion 62 causes the sealing surface 45 to become irregular (not illustrated) thus a closure (not illustrated) subsequently attached to the neck-finish 44 will not properly seal.
To assure that the polymer within the wall thickness 56 is sufficiently solid and rigid to transmit forces applied by the neck-ring 18a and 18b, without neck-finish distortion occurring during removal, molding technicians may extend the time to manufacture the preform 26. Typical molding time needed for manufacturing the preform 26 of thermoplastic polyester materials is about 21 to 26 seconds. An attempt to alleviate this problem was made in another early Husky design wherein a small portion of the neck ring (less than fifty percent) was made to contact an outer circumferential portion of the top sealing surface of the preform. However, this design suffered from two disadvantages. First the small area of contact between the neck ring and the top sealing surface still required substantial cooling time to prevent neck ring distortions. Second, this design had the cylindrical neck ring mating surfaces which allowed for leakage of the molten plastic.
U.S. Pat. Nos. 4,521,177; 6,176,700; 6,220,850 and 6,413,075 show insert assembly arrangements for molding preforms. U.S. Pat. Nos. 4,025,022; 4,125,246; 4,179,254; 4,632,357; 4,648,834; and 5,137,442 show other injection molding machines utilizing various stripping devices. Also, U.S. patent application Ser. No. 10/350,325, filed Jan. 24, 2003, describes method and apparatus whereby a lifting structure lifts the preform by an edge of the open end thereof. Since the molded plastic article is lifted by its end, the article does not have to be solidified at its interior, thus allowing earlier removal of the article from the mold, reducing cycle time. However, even faster stripping times are desirable.
Therefore, there is a need for a neck finish portion cooling method and apparatus, which provides rapid, efficient neck cooling while further reducing the molding cycle time to further decrease the cost of producing molded plastic preforms.